The presence of putative erq proteins encoded by the erq (ets- related gene) locus has been examined in specific human cell lines. To probe the erq proteins, polyclonal antisera have been prepared against synthetic polypeptides representing amino acid sequences deduced from the DNA sequence of the erg gene locus. Furthermore, we have to facilitate further analysis of the putative erg proteins; we have purified a bacterially expressed erg protein and are using this highly purified protein to raise polyclonal erg antiserum. In immunoprecipitates from the nuclear fractions of 35S-methionine-labeled COLO 320 cells we detect two proteins of 56 and 63 Kd which reacted specifically with the specific erg-RI antiserum. Whether these proteins represent the cellular erq gene products is currently under investigation.